Fuel-cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
Fuel cells include three components: a cathode, an anode and an electrolyte, which is sandwiched between the cathode and the anode. Each electrode is coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.
In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. The polymer electrode membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate conductivity of the membrane. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is very important for the proper functioning of the fuel cell. Irreversible damage to the fuel cell will occur if the membrane dries out.
In order to prevent leakage of the hydrogen fuel gas and oxygen gas supplied to the electrodes and prevent mixing of the gases, a gas-sealing material and gaskets are arranged on the periphery of the electrodes, with the polymer electrolyte membrane sandwiched there between. The sealing material and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane to form a membrane and electrode assembly (MEA). Disposed outside of the MEA are conductive separator plates for mechanically securing the MEA and electrically connecting adjacent MEAs in series. A portion of the separator plate, which is disposed in contact with the MEA, is provided with a gas passage for supplying hydrogen fuel gas to the electrode surface and removing generated water vapor.
During fabrication of a fuel cell, the polymer electrolyte membrane of each MEA is produced in roll form under tension. The polymer electrolyte membrane has a high water uptake capability. Therefore, when wet, the membrane will expand in all three directions, although not proportionally. The membrane will shrink in all three dimensions upon subsequent drying.
Because the proton conductivity of PEM fuel cell membranes deteriorates rapidly as the membranes dry out, external humidification is required to maintain hydration of the membranes and sustain proper fuel cell functioning. Moreover, the presence of liquid water in automotive fuel cells is unavoidable because appreciable quantities of water are generated as a by-product of the electrochemical reactions during fuel cell operation. Furthermore, saturation of the fuel cell membranes with water can result from rapid changes in temperature, relative humidity, and operating and shutdown conditions. However, excessive membrane hydration results in flooding, excessive swelling of the membranes and the formation of differential pressure gradients across the fuel cell stack.
In order to maintain consistent fuel cell stack pressures, membranes are needed which swell uniformly and then only marginally in the presence of liquid water. Perfluorosulfonic acid (PFSA) membranes are typically used because of their advantaged oxidative, chemical and thermal stability and because of their superior proton conductivities at low relative humidity. PFSA membranes with a wide range of physical properties are available, and performance depends on the membrane's ion exchange capacity and the internal stresses and defects introduced during the membrane-film preparation process.
PFSA membranes with high acid numbers (or low equivalent weights) have enhanced proton conductivity at reduced relative humidity, but the mechanical properties of these membranes (especially with the high acid numbers) are compromised because of swelling due to high water uptake. The in-plane swelling of extruded membranes is further complicated because the membranes typically swell less in the machine process direction (the x-axis) as compared to the transverse direction (the y-axis) of the film. Non-uniform membrane swelling introduces the possibility of uneven, pressure-related stress failure mechanisms in fuel cell stacks.
Non-uniform membrane swelling is shown in FIG. 1, in which a membrane film 16 is conventionally cut from a membrane sheet 10 in such an orientation that the side edges 18 of the membrane film 16 are parallel to the machine process direction 12 and the transverse edges 20 of the membrane 16 are parallel to the transverse direction 14 of the membrane sheet 10. As shown in FIG. 1A, after immersion in water or ionomer solution, the swelled membrane film 16a expands to a greater extent along the transverse edges 20 than along the side edges 18. The side edges 18 of the pre-immersed membrane film 16a are indicated by the dashed lines in FIG. 1A.
One method used to make the membrane films swell more uniformly and reduce internal stress is to prepare membrane films by solution-casting rather than by extrusion-processing. Solvent cast films are expected to swell more uniformly and to have less internal stresses as compared with those of extruded films. Another approach is to reinforce the ionomer by incorporating it into a non-swelling support structure. This method is practiced by W. L. Gore, Inc., which markets composite membranes made with low equivalent weight, PFSA ionomer that is imbibed into a porous, expanded polytetrafluoroethylene support matrix. The structural strength of the membrane is reinforced and uniform swelling is maintained by the polytetrafluoroethylene support structure.
A novel method to improve the swelling uniformity and improve the mechanical strengths of extruded PFSA membranes has been found. This method includes cutting the membrane film from a sheet of the extruded membrane film in a diagonal orientation, such that membrane swelling or expansion becomes more uniform in the x and y directions. Drying of the swollen membrane film under tension, followed by re-cutting of the expanded membrane film, produces a functional polymer electrolyte membrane having a tendency to expand more uniformly in the x and y directions. Thus, internal stresses within the membrane are relieved throughout wet/dry cycles during functioning of the fuel cell.